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The 1918 flu pandemic (commonly referred to as the Spanish flu) was an influenza pandemic that spread to nearly every part of the world. It was caused by an unusually virulent and deadly Influenza A virus strain of subtype H1N1. Historical and epidemiological data are inadequate to identify the geographic origin of the virus. Most of its victims were healthy young adults, in contrast to most influenza outbreaks which predominantly affect juvenile, elderly, or otherwise weakened patients. The flu pandemic has also been implicated in the sudden outbreak of Encephalitis lethargica in the 1920s.

The pandemic lasted from March 1918 to June 1920, spreading even to the Arctic and remote Pacific islands. It is estimated that anywhere from 50 to 100 million people were killed worldwide, or the approximate equivalent of one third of the population of Europe. An estimated one third of the world (500 million people) became infected.

Although the first cases of the disease were registered in the continental US and the rest of Europe long before getting to Spain, the 1918 Flu received its nickname "Spanish Flu" because Spain, a neutral country in WWI, had no special censorship for news against the disease and its consequences. Hence the most reliable news came from Spain, giving the false impression that Spain was the most—if not the only—affected zone.

Scientists have used tissue samples from frozen victims to reproduce the virus for study. Given the strain's extreme virulence there has been controversy regarding the wisdom of such research. Among the conclusions of this research is that the virus kills via a cytokine storm (overreaction of the body's immune system) which explains its unusually severe nature and the concentrated age profile of its victims. The strong immune systems of young adults ravaged the body, whereas the weaker immune systems of children and middle-aged adults caused fewer deaths.